1. Field of the Invention
The present invention relates to a microelectromechanical system, in which a transducer and an interconnect portion for electrically connecting the transducer to an external device are integrated together on the same substrate, and also relates to a method for fabricating such a system.
2. Description of the Related Art
Microelectromechanical systems (MEMS), including a transducer and a controller that are integrated together on the same substrate, have recently been researched and developed extensively. As used herein, the “transducer” means an element having the function of converting electrical energy into energy in another form and/or vice versa. A micro actuator and a micro sensor, at least part of which is a physically deformable structure, are typical transducers for converting electrical energy into mechanical energy.
A MEMS of that type is fabricated by providing a controller on a substrate first and then forming a transducer on the same substrate. Accordingly, if the process step of forming the transducer must be carried out at an elevated temperature, then the controller may deteriorate due to the heat, which is a problem.
Generally speaking, the transducer needs to exceed a standard level in terms of residual stress, creep strength, abrasion resistance, fatigue failure resistance and surface roughness. A normal LSI fabricated by a semiconductor device manufacturing process needs to exhibit none of these mechanical properties. To obtain a transducer with these mechanical properties improved, thin film deposition and annealing process steps, which should be carried out at high temperatures, are often essential parts of its manufacturing process. For example, polysilicon, which is now being researched most extensively as one of the most suitable materials for a MEMS, needs to be annealed at least at 600° C. for 150 minutes, or at 1,050° C. for 30 minutes, to obtain a film of quality with little residual stress. It is known that the residual stress of a silicon nitride can be reduced to 10 MPa or less if the silicon nitride has a silicon-rich composition and if the silicon nitride is annealed within an oxygen atmosphere. In that case, however, the silicon nitride still needs to be annealed at a temperature of 825° C. to 850° C. for at least 180 minutes. Also, lead zirconate titanate (PZT), which is a typical ferroelectric oxide, needs to be thermally treated at temperature of 600° C. to 650° C. to align crystalline phases of PZT. And a silicon carbide, having extremely high mechanical-chemical strength, has to be deposited at a temperature of 750° C. to 800° C. As is well known in the art, polysilicon, silicon nitride, lead zirconate titanate, and silicon carbide have excellent material properties and are often used in semiconductor device processing but need to be thermally treated at least at 600° C. when used as a material for a good transducer structure.
Meanwhile, the controller of a normal CMOS circuit has a thermal resistance of at most about 450° C. to about 525° C. Accordingly, if the transducer, requiring such a high-temperature process, were formed after the controller has been provided, then the controller might be damaged.
It is known that the controller is damaged mainly because an aluminum alloy, which is usually used in the interconnect portion thereof, increases its resistance so much as to be disconnected. Aluminum has as low a melting point as 660° C. Accordingly, once the controller including aluminum or an alloy thereof is complete, no high temperature process may be carried out at a temperature of about 660° C. or more. In addition, even at a temperature lower than 660° C., aluminum is likely to create voids due to grain growth or atomic diffusion. Thus, if the interconnects of the controller are made of aluminum with such a low melting point, then the interconnects would deteriorate while the transducer is being fabricated. For that reason, when aluminum is used as an interconnect material for the controller, the transducer needs to be fabricated in such a temperature range that would not deteriorate the aluminum interconnects. That is to say, the temperature at which the aluminum interconnects start to deteriorate defines the thermal resistance of the controller.
To increase the thermal resistance of the controller, aluminum may be replaced with tungsten, which is a refractory metal well known in the field of semiconductor integrated circuit technologies (see J. M. Bustillo, R. T. Howe and R. S. Muller, “Surface Micromachining for Microelectromechanical Systems”, Proceedings of the IEEE, Vol. 86, No. 8, pp. 1552–1574 (August 1998)(FIG. 4, in particular)). Bustillo et al. discloses a MEMS including a controller, in which the conductor is made of tungsten, and a transducer including a polysilicon portion. In this MEMS, polysilicon as the transducer material is annealed at as high a temperature as 1,050° C. for one hour and the controller can resist such a high-temperature process.
Meanwhile, in manufacturing a normal LSI, a damascene process, in which excessive portions of a conductor are removed by chemical mechanical polishing (CMP), is used extensively today. When this damascene process is adopted, a copper-based material, which has been too difficult to etch to adopt in semiconductor device processing, can be used. The damascene process makes it possible to form very small interconnects without being affected by the topology of lower-level interconnects. Copper has lower resistivity than aluminum. Accordingly, by using copper interconnects, the electrical resistance of the interconnects can be reduced and the operating frequency of the CPU can be increased.
The copper interconnect includes a barrier layer of titanium nitride to prevent the diffusion of copper atoms. The critical thickness of the barrier layer is defined such that the resistivity of the copper interconnect with the barrier layer is lower than that of the aluminum interconnect without the barrier layer. In this case, the resistivity of aluminum is 1.58 times as high as that of copper. In the prior art, however, the copper interconnects obtained by the damascene process are used only as very small interconnects that satisfy a design rule of 0.25 μm, and therefore, the barrier layer thereof needs to have a thickness of 40 nm or less, which is an extremely small value.
It is also known that when 2.98 wt % of titanium is added to copper, the grain growth of the copper can be minimized even during an annealing process at 800° C. (see, for example, C. J. Liu, J. S. Jeng and J. S. Chen, “Effects of Ti addition on the morphology, interfacial reaction, and diffusion of Cu on SiO2”, Journal of Vacuum Science & Technology B, Vol. 20, No. 6, pp. 2361–2366 (November/December 2002)).
The damascene process is sometimes applied to micro-machining (see, for example, H. Lakdawala et al., “Micromachined High-Q Inductors in a 0.18-μm Copper Interconnect Low-K Dielectric CMOS Process”, IEEE Journal of Solid-State Circuits, Vol. 37, No. 3, pp. 394–403 (March 2002)). Lakdawala et al. discloses a configuration in which a stack of 0.18 μm copper interconnects and a low-k interlayer dielectric film is used as an inductor, the low-k interlayer dielectric film is removed by an anisotropic etching process with the uppermost metal layer used as a mask, and then the substrate is selectively etched isotropically to define a hollow structure under the inductor.
In the configuration disclosed by Bustillo et al., however, tungsten as the conductor has high resistance, which also varies greatly with the temperature. Accordingly, the loss and resistance variation of a long interconnect are particularly significant. As a result, the operating speed of the MEMS decreases, the power dissipation thereof increases or the operating characteristics of the element change with the temperature characteristic of the interconnects.
As for the copper interconnects for use in a normal LSI, the diffusion of copper atoms needs to be prevented with the barrier layer that is as thin as 40 nm or less. Accordingly, strict process control must be carried out to adapt the copper interconnects to a high-temperature long process. The diffusion path of the copper atoms in the barrier layer is mainly located in the grain boundary of the barrier material. For that reason, to enhance the anti-diffusion effects, it is particularly effective to increase the grain sizes of the barrier material and align the crystal directions between the grains. However, when the barrier layer is thin, most of the atoms in the barrier layer are subject to interfacial effects produced between the barrier layer and the insulating film, and therefore, tend to have decreased grain sizes and randomized crystal directions. To avoid these situations with good reliability, strict control and test of the interconnect quality are needed.
Also, if about 2.98 wt % of titanium is added to copper to minimize the grain growth of that copper, then the specific resistance of the copper becomes 5 μΩcm, which is greater than the specific resistance of aluminum of 2.7 μΩcm. Thus, this technique does not contribute effectively to achieving a lower resistance than that of aluminum.
Lakdawala et al. proposed that the damascene process be applied to making a micro machine. According to his method, the inductor corresponding to the transducer is formed by a low-temperature process. But Lakdawala et al. is silent about what configuration should be used when the transducer needs a high-temperature process. Also, according to the technique of Lakdawala et al., the interlayer dielectric film is patterned by an anisotropic etching process, and then the silicon substrate is selectively etched isotropically to define a hollow structure. Thus, only transducers with a very simple structure can be obtained. In addition, the portions of the silicon substrate to be etched away cannot be fixed definitely. Consequently, it is difficult to prevent the controller from being damaged by over-etching or the transducer from losing its reliability due to under-etching.